diff --git a/plugin/federated/CMakeLists.txt b/plugin/federated/CMakeLists.txt
index 24ba47abfb8e..8dbf8227f887 100644
--- a/plugin/federated/CMakeLists.txt
+++ b/plugin/federated/CMakeLists.txt
@@ -7,7 +7,7 @@ find_package(Threads)
 add_library(federated_proto federated.proto)
 target_link_libraries(federated_proto PUBLIC protobuf::libprotobuf gRPC::grpc gRPC::grpc++)
 target_include_directories(federated_proto PUBLIC ${CMAKE_CURRENT_BINARY_DIR})
-set_property(TARGET federated_proto PROPERTY POSITION_INDEPENDENT_CODE ON)
+xgboost_target_properties(federated_proto)
 
 get_target_property(grpc_cpp_plugin_location gRPC::grpc_cpp_plugin LOCATION)
 protobuf_generate(TARGET federated_proto LANGUAGE cpp)
diff --git a/plugin/federated/federated_communicator.h b/plugin/federated/federated_communicator.h
index cb1eb0b8109f..2dc50962a296 100644
--- a/plugin/federated/federated_communicator.h
+++ b/plugin/federated/federated_communicator.h
@@ -4,6 +4,7 @@
 #pragma once
 #include <xgboost/json.h>
 
+#include "../../src/c_api/c_api_utils.h"
 #include "../../src/collective/communicator.h"
 #include "../../src/common/io.h"
 #include "federated_client.h"
@@ -89,31 +90,14 @@ class FederatedCommunicator : public Communicator {
       client_cert = value;
     }
 
-    // Runtime configuration overrides.
-    auto const &j_server_address = config["federated_server_address"];
-    if (IsA<String const>(j_server_address)) {
-      server_address = get<String const>(j_server_address);
-    }
-    auto const &j_world_size = config["federated_world_size"];
-    if (IsA<Integer const>(j_world_size)) {
-      world_size = static_cast<int>(get<Integer const>(j_world_size));
-    }
-    auto const &j_rank = config["federated_rank"];
-    if (IsA<Integer const>(j_rank)) {
-      rank = static_cast<int>(get<Integer const>(j_rank));
-    }
-    auto const &j_server_cert = config["federated_server_cert"];
-    if (IsA<String const>(j_server_cert)) {
-      server_cert = get<String const>(j_server_cert);
-    }
-    auto const &j_client_key = config["federated_client_key"];
-    if (IsA<String const>(j_client_key)) {
-      client_key = get<String const>(j_client_key);
-    }
-    auto const &j_client_cert = config["federated_client_cert"];
-    if (IsA<String const>(j_client_cert)) {
-      client_cert = get<String const>(j_client_cert);
-    }
+    // Runtime configuration overrides, optional as users can specify them as env vars.
+    server_address = OptionalArg<String>(config, "federated_server_address", server_address);
+    world_size =
+        OptionalArg<Integer>(config, "federated_world_size", static_cast<Integer::Int>(world_size));
+    rank = OptionalArg<Integer>(config, "federated_rank", static_cast<Integer::Int>(rank));
+    server_cert = OptionalArg<String>(config, "federated_server_cert", server_cert);
+    client_key = OptionalArg<String>(config, "federated_client_key", client_key);
+    client_cert = OptionalArg<String>(config, "federated_client_cert", client_cert);
 
     if (server_address.empty()) {
       LOG(FATAL) << "Federated server address must be set.";
diff --git a/src/c_api/c_api_utils.h b/src/c_api/c_api_utils.h
index ba23f765e641..b7b2c93034d3 100644
--- a/src/c_api/c_api_utils.h
+++ b/src/c_api/c_api_utils.h
@@ -248,13 +248,23 @@ inline void GenerateFeatureMap(Learner const *learner,
 
 void XGBBuildInfoDevice(Json* p_info);
 
+template <typename JT>
+void TypeCheck(Json const &value, StringView name) {
+  using T = std::remove_const_t<JT> const;
+  if (!IsA<T>(value)) {
+    LOG(FATAL) << "Incorrect type for: `" << name << "`, expecting: `" << T{}.TypeStr()
+               << "`, got: `" << value.GetValue().TypeStr() << "`.";
+  }
+}
+
 template <typename JT>
 auto const &RequiredArg(Json const &in, std::string const &key, StringView func) {
   auto const &obj = get<Object const>(in);
   auto it = obj.find(key);
   if (it == obj.cend() || IsA<Null>(it->second)) {
-    LOG(FATAL) << "Argument `" << key << "` is required for `" << func << "`";
+    LOG(FATAL) << "Argument `" << key << "` is required for `" << func << "`.";
   }
+  TypeCheck<JT>(it->second, StringView{key});
   return get<std::remove_const_t<JT> const>(it->second);
 }
 
@@ -262,7 +272,8 @@ template <typename JT, typename T>
 auto const &OptionalArg(Json const &in, std::string const &key, T const &dft) {
   auto const &obj = get<Object const>(in);
   auto it = obj.find(key);
-  if (it != obj.cend()) {
+  if (it != obj.cend() && !IsA<Null>(it->second)) {
+    TypeCheck<JT>(it->second, StringView{key});
     return get<std::remove_const_t<JT> const>(it->second);
   }
   return dft;
diff --git a/src/collective/noop_communicator.h b/src/collective/noop_communicator.h
index 5d4fb89f409c..cad6da029530 100644
--- a/src/collective/noop_communicator.h
+++ b/src/collective/noop_communicator.h
@@ -17,9 +17,8 @@ class NoOpCommunicator : public Communicator {
   NoOpCommunicator() : Communicator(1, 0) {}
   bool IsDistributed() const override { return false; }
   bool IsFederated() const override { return false; }
-  void AllReduce(void *send_receive_buffer, std::size_t count, DataType data_type,
-                 Operation op) override {}
-  void Broadcast(void *send_receive_buffer, std::size_t size, int root) override {}
+  void AllReduce(void *, std::size_t, DataType, Operation) override {}
+  void Broadcast(void *, std::size_t, int) override {}
   std::string GetProcessorName() override { return ""; }
   void Print(const std::string &message) override { LOG(CONSOLE) << message; }
 
diff --git a/tests/cpp/c_api/test_c_api.cc b/tests/cpp/c_api/test_c_api.cc
index 30e2b5cd0475..ea1f893164f8 100644
--- a/tests/cpp/c_api/test_c_api.cc
+++ b/tests/cpp/c_api/test_c_api.cc
@@ -324,4 +324,36 @@ TEST(CAPI, NullPtr) {
   ASSERT_NE(pos, std::string::npos);
   XGBAPISetLastError("");
 }
+
+TEST(CAPI, JArgs) {
+  {
+    Json args{Object{}};
+    args["key"] = String{"value"};
+    args["null"] = Null{};
+    auto value = OptionalArg<String>(args, "key", std::string{"foo"});
+    ASSERT_EQ(value, "value");
+    value = OptionalArg<String const>(args, "key", std::string{"foo"});
+    ASSERT_EQ(value, "value");
+
+    ASSERT_THROW({ OptionalArg<Number>(args, "key", 0.0f); }, dmlc::Error);
+    value = OptionalArg<String const>(args, "bar", std::string{"foo"});
+    ASSERT_EQ(value, "foo");
+    value = OptionalArg<String const>(args, "null", std::string{"foo"});
+    ASSERT_EQ(value, "foo");
+  }
+
+  {
+    Json args{Object{}};
+    args["key"] = String{"value"};
+    args["null"] = Null{};
+    auto value = RequiredArg<String>(args, "key", __func__);
+    ASSERT_EQ(value, "value");
+    value = RequiredArg<String const>(args, "key", __func__);
+    ASSERT_EQ(value, "value");
+
+    ASSERT_THROW({ RequiredArg<Integer>(args, "key", __func__); }, dmlc::Error);
+    ASSERT_THROW({ RequiredArg<String const>(args, "foo", __func__); }, dmlc::Error);
+    ASSERT_THROW({ RequiredArg<String>(args, "null", __func__); }, dmlc::Error);
+  }
+}
 }  // namespace xgboost
diff --git a/tests/cpp/plugin/test_federated_communicator.cc b/tests/cpp/plugin/test_federated_communicator.cc
index 2d9f233db573..3cfa15fb1eb7 100644
--- a/tests/cpp/plugin/test_federated_communicator.cc
+++ b/tests/cpp/plugin/test_federated_communicator.cc
@@ -85,6 +85,28 @@ TEST(FederatedCommunicatorSimpleTest, ThrowOnRankTooBig) {
   EXPECT_THROW(construct(), dmlc::Error);
 }
 
+TEST(FederatedCommunicatorSimpleTest, ThrowOnWorldSizeNotInteger) {
+  auto construct = []() {
+    Json config{JsonObject()};
+    config["federated_server_address"] = kServerAddress;
+    config["federated_world_size"] = std::string("1");
+    config["federated_rank"] = Integer(0);
+    auto *comm = FederatedCommunicator::Create(config);
+  };
+  EXPECT_THROW(construct(), dmlc::Error);
+}
+
+TEST(FederatedCommunicatorSimpleTest, ThrowOnRankNotInteger) {
+  auto construct = []() {
+    Json config{JsonObject()};
+    config["federated_server_address"] = kServerAddress;
+    config["federated_world_size"] = 1;
+    config["federated_rank"] = std::string("0");
+    auto *comm = FederatedCommunicator::Create(config);
+  };
+  EXPECT_THROW(construct(), dmlc::Error);
+}
+
 TEST(FederatedCommunicatorSimpleTest, GetWorldSizeAndRank) {
   FederatedCommunicator comm{6, 3, kServerAddress};
   EXPECT_EQ(comm.GetWorldSize(), 6);